You can gift purchased games to any person with GOG account - visit the page of game you want to give away, and there under 'you already own it' banner is the option 'gift it'.
That involves re-buying the game though, it does not allow you to give away a game you already own. Frankly, there is no way to do that on GOG, but it is definitely the kind of thing that is worth suggesting to GOG as a future feature.
Also, I don't think it will ever be possible, due to the fact that GOGs are DRM-free. Even if you give your game away from your account, you'll be able to play it. I know, they trust us and all, but they would be overly idealistic to believe someone really
deleted all traces of the game's executable from every place he/she made a backup of it after selling/giving it away.